Solenoid drive circuit

ABSTRACT

A circuit for initially applying an unusually large drive voltage to a solenoid coil and for subsequently reducing the applied voltage during the travel of the solenoid plunger. The solenoid coil is serially connected to a first transistor circuit operating as an on-off switch and also is serially connected to a second transistor circuit operating to variably control the voltage applied to the solenoid. A capacitor-charge timing circuit controls the variable transistor and thereby gradually reduces the voltage applied to the solenoid.

United States Patent Mason 14 1 May 2, 1972 [541 SOLENOID DRIVE CIRCUIT 3,200,308 8/1965 Mazgy ..3 17/010. 4 3,205,412 9/1965 Winston ..3 l 7/DIG. 4 [72] Hamsbmg Ohm 3,206,651 9/1965 Proulx .317/1310. 4 [73] Assignee: Design Elements Inc.

. Primary Examiner-L. T. Nix [22] Ffled' Dec. 1970 AttorneyAnthony D. Cennamo [21] Appl. No.: 98,627

[57] ABSTRACT [52] US. Cl. ..3l7/148.5, 317/154, 3l7/DIG. 4 A i i f initially l i an unusually large drive voltage l Cl ---H01h47/32, H011 47/04 to a solenoid coil and for subsequently reducing the applied [58] Fleld of Search ..3l7/DlG. 4, 148.5, 154 voltage during the travel of the Solenoid plunger The Solenoid coil is serially connected to a first transistor circuit operating [56] References cued as an on-off switch and also is serially connected to a second UNITED STATES PATENTS transistor circuit operating to variably control the yoltage ap plied to the solenoid. A capacltor-charge tlmmg clrcuit con 3,084,310 4/ 1963 Schurr ..317/DIG. 4 1 h i bl i tor and thereby gradually reduces the 3 ,02 l l Pickens 4 voltage to the solenoid 3,064,165 l 1/1962 Kennedy... ....3l7/DlG. 4 3,116,441 12/1963 Gieffers ..3 l 7/DIG. 4 10 Claims, 2 Drawing Figures R7 I R6\ Patented May 2,1972 7 -3, 660,730

INVENTOR. EDWIN EMASON v ATTORNEY BACKGROUND OF THE INVENTION This invention relates to electromechanical control systems and more particularly relates to an electronic circuit for applying a desired voltage waveform to a solenoid coil.

The solenoid is an old and well established electromechanical device. It is used in diverse applications for changing electrical energy to mechanical energy. For example, I am interested primarily in the uses of solenoids for operating an input-output typewriter in response to remotely originated electrical signals in the manner disclosed in my copending application, Ser. No., 79,202, filed Oct. 8, 1970, for Input-Output Typewriter Apparatus," by Edwin E. Mason, and assigned to the same assignee as the instant application.

The conventional manner of operating a solenoid is to apply to the solenoid a voltage which is slightly larger than or equal to the minimum voltage necessary to actuate the solenoid. Usually this voltage is continuously applied to the solenoid until the solenoid is de-energized. To deenergize a solenoid, the voltage is lowered to a value below the minimum holding voltage and ordinarily to zero volts.

There are several undesirable consequences of this conventional operation which my invention overcomes. With all solenoids, a minimum actuation voltage is necessary to overcome the inertia of the solenoid and to begin the movement of its plunger. However, with the continued application of the same voltage, the plunger will accelerate until its travel is abruptly ended by a mechanical stop. At the instant the plunger strikes the mechanical stop, it is traveling at its maximum velocity and consequently has its maximum momentum. The sudden stopping of the plunger produces a sudden slamming or hammering of the plunger into the mechanical parts of the solenoid. This hard slamming of the plunger tends to jar all the equipment with which the solenoid is associated and in particular tends to weaken, wear, and eventually damage the solenoid.

Because a lower voltage can be utilized to hold the solenoid in its actuated position after the plunger comes to rest, the continued application of the higher actuation voltage functions only to waste electrical power and to cause the dissipation of excess heat into the associated equipment.

There is a need therefore, for a solenoid drive circuit which will initially overdrive the solenoid by applying a voltage to the solenoid which is considerably greater than its minimum actuation voltage. This will hasten the change in momentun of the plunger and similarly will more rapidly raise the solenoid current to the actuating current by more quickly overcoming the high initial reactance of the solenoid. There is also a need for a circuit which will automatically reduce the voltage applied to the solenoid during the plunger travel after the solenoid current has risen to a desired value, and yet will continue to maintain the solenoid current slightly above or equal to the minimum holding current of the solenoid.

SUMMARY OF THE INVENTION The invention is a solenoid drive circuit for energizing and for de-energizing a solenoid in response to suitable signals at a drive circuit input. The solenoid drive circuit has a first electronic switch means which is in series with a solenoid and a source of driving electrical power. The first electronic switch means switches the current to the solenoid and includes the current for controlling the valve means. The timing means comprises serially connected timing resistance, capacitance and a diode connected to permit current flow charging the capacitance. The timing means is connected between the control input of the valve means and the other solenoid terminal. Discharge means for discharging the capacitance is also provided. The discharge means comprises a serially connected dissipating resistance and a second electronic switch means connected across the capacitance. The control input of the second electronic switch is connected to have at least the voltage across the diode applied to the control input of the second electronic switch.

OBJECTS OF THE INVENTION It is an object of the invention to provide an improved solenoid drive circuit.

Another object of the invention is to provide a solenoid drive circuit which will initially overdrive the solenoid until the solenoid is driven at approximately its minimum holding current.

Another object of the invention is to speed up the actuation and complete operation of a solenoid and at the same time reduce the momentum of the solenoid plunger as it arrives at its actuated position.

Another object of the invention is to provide a solenoid drive circuit which will exhibit an improved duty cycle so that solenoid operation may be improved while heat generation and power losses are minimized.

Further objects and features of the invention will be apparent from the following specification and claims when considered in connection with the accompanying drawings illustrating the invention.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic view of the preferred embodiment of the invention; and

FIG. 2 is a graph illustrating solenoid voltage and current during operation of the solenoid.

In describing the preferred embodiment of the invention illustrated in the drawings, specific terminology will be resorted to for the sake of clarity. However, it is not intended to be limited to the specific terms so elected and it is to be understood that each specific ten-n includes all technical equivalents which operates in a similar manner to accomplish a similar purpose.

DETAILED DESCRIPTION The preferred embodiment of the invention is illustrated in FIG. 1. The solenoid 10 is to be controlled by the remainder of the circuit which receives its driving power from a source at the terminals 12 and 14. The circuitry of the section 16 outlined in broken lines is simply a carlington pair connected and operated as a fust electronic switch means 16. The electronic switch means 16 includes and is'controlled at its drive circuit input 18. It is intended that the drive circuit be digitally controlled and therefore no current will flow through the darlington pair when the input is at zero volts while current can flow through the darlington pair when the input voltage is at the suitable positive voltage. Thus the electronic switch 16 is closed when zero volts is applied at the input terminal 18 and is opened when a positive voltage is applied at the terminal 18. The function therefore, of the electronic switch 16, is to switch the current through the solenoid 10.

The solenoid 10 is also serially connected to an electronic valve means which, in the embodiment illustrated in FIG. 1, is the bipolar transistor 0,. Transistor O is connected in a common emitter configuration and has a control input at its base terminal 20. The purpose of the transistor O is to varibly control a portion of the solenoid current. It will become obvious that many other electronic valve means could be used in place of Q and specifically a field effect transistor may be highly desirable in some applications.

A resistor R is connected to bypass solenoid current around Q, and therefore is connected parallel to the valve means 0,. The purpose of the resistor R is to permit a minimum holding current to flow through the solenoid when the transistor Q, is not conducting.

The preferred value of the resistor R in the circuit illustrated in FIG. 1 is 220 ohms at 3 watts. The important fact is that the value is selected in order to provide the minimum holding current and therefore the value of R is approximately equal to E /I R,,,'where R, is the solenoid DC resistance, E is the voltage of the source of power at the terminals 12 and 14, and I,, is the minimum solenoid holding current.

The electronic valve transistor Q, is controlled by a timing means indicated generally as 30. The timing means comprises a serially connected timing resistance R,,, a capacitance C, and a diode D, which is connected at a polarity to permit current flow charging the capacitor. This series connected timing means is connected between the control input at the base of transistor Q and the terminal of the solenoid to which 0., is not connected. The function of the timing means is to provide a time changing current for controlling the valve means transistor 0,. The charging circuit of the timing means 30 is through the base-emitter junction of the transistor 0,, through the series connected timing means 30 and through the electronic switch 16. A leakage resistor R, is connected between the base 20 of the transistor Q, and the power source terminal 12 in order to provide a path for leakage current through the base when the transistor 0., is cut off.

As will be seen below, the capacitor C, charges during the excitation of the solenoid 10 and must be discharged when the solenoid 10 is de-energized. For this purpose a capacitance discharge means 31 is provided by a series connected resistor R and a second electronic switch in the form of the transistor 0,. These are connected across the capacitance C, so that when the transistor Q, is conducting the capacitance C, will be discharged. The base of the transistor Os, which functions as the control input of this second electronic switch, is connected to have at least the voltage appearing across the diode applied to the base of the transistor Q,. In the preferred circuit the capacitor C, and the diode D, share a common node 34 to which the emitter of the transistor 0, is connected. The base is connected to the opposite side of the diode D, so that the diode voltage appears at the base to emitter input of the transistor Q OPERATION Referring now to FIGS. 1 and 2, we may assume that operation begins with the solenoid de-energized and with zero volts appearing at the input 18 so that no current is flowing through the electronic switch 16. We shall assume also that the capacitor C, is fully discharged.

When an input signal appears at the input terminal 18 in the form of a positive step function, the darlington pair of the electronic switch 16 will begin conducting. Immediately, as illustrated at time zero in FIG. 2, the voltage across the solenoid will be substantially the power supply voltage at the terminals 12 and 14 (more exactly it will be less by the sum of the emitter-collector voltages of transistors Q, and 0,). For purposes of illustration we have selected this voltage V, as 40 volts. The positive input signal voltage will continue at the input 18 so long as it is desired to hold the solenoid in an energized position. The current i, in the solenoid 10 (shown in FIG. 2) will begin to increase in approximately the ordinary exponential manner. Simultaneously, current will begin flowing through the emitter-base junction of the transistor 0, and through the timing means 30 and will begin charging the capacitor C,. Eventually, for example at T in FIG. 2, the capacitor will be fully charged such that base current no longer flows in transistor 0,. At this point transistor Q, will cease conducting and the voltage on the solenoid will be substantially reduced to a minimum holding level, for example, V, illustrated in FIG. 2. Only a minimum holding current will be flowing through the solenoid and through the resistor R All during the time these things were happening, the voltage on the diode D, will have caused the diode to be forward biased and therefore will maintain the discharging transistor 0 in a nonconducting state. As long as the appropriate positive input appears at the terminal 18, the solenoid will remain energized with the minimum holding current flowing through it.

In some solenoid applications there is no need to hold the solenoid plunger in its actuated position. Often a designer desires only to actuate the plunger and then immediately return it to the de-energized position. Such operation is easily accomplished while maintaining the advantages of this invention. The holding current resistance R, is merely eliminated from the circuit. Removal of this current path which is parallel to the valve means of transistor Q, eliminates any holding current. Theoretically, R is made to have infinite value to provide zero holding current. With this modification, the voltage applied to the solenoid will taper to zero volts instead of to a minimum holding voltage, and the solenoid current will similarly taper to zero amps. The solenoid will be smoothly actuated and then the plunger will return to the de-energized position when the solenoid current tapers below the minimum holding current. The time this takes and the voltage and current taper can be selected by design as described below.

If now the input voltage is reduced to zero, current will no longer be able to flow through the solenoid 10 from the power supply at the terminals 12 and 14. At this instant of time indicated as T in FIG. 2, the magnetic field of the solenoid will begin to collapse and the voltage polarity of the solenoid will reverse. The reversed solenoid voltage will be applied in a back biased direction across the diode D, and will therefore immediately turn on the transistor Q, and permit quick discharging of the capacitor C,. This will continue until the energy stored in the solenoid 10 and in the capacitance C, is completely dissipated in the resistances of the circuit. At that point the circuit has returned to its initial de-energized condition.

Referring now to FIG. 2 it is obvious that the time constant of the timing circuit 30 can be controlled and therefore proper circuit design permits selection of a difierent variety of waveforms and duty cycle conditions on the solenoid 10.

If, for example, R, is made relatively small to provide a shorter time constant and if R, is chosen so that, at the beginning of capacitor C, charging, the transistor begins operating well in the conducting region way above the transistor linear region, then a waveform like that indicated as V, in FIG. 2 will be provided.

If R, is made relatively large for a longer time constant to produce a nearly linear ramp function, and if R, is made such that capacitor C, charge begins with the transistor operation on the edge of the linear region, a drive voltage like V, will occur.

Of course a continuous family of solenoid drive voltage waveforms exists. A designer can select the waveform which meets the mechanical and the electrical characteristics of the solenoid. For example, a quicker acting, lower momentum solenoid would utilize a waveform like V The circuit values are also chosen with a consideration of the time needed for the plunger of the solenoid to complete its travel. Solenoid travel could end at T,, T,, or T,,, although I prefer it end near time T,. For example, in the preferred circuit illustrated, a time in the range of 8 to 14 milliseconds as T, has given desirable results.

The improved results of the invention become even more advantageous when used with devices such as a computer operated typewriter. Such a typewriter will be operated as rapidly as possible. This means that the solenoids will average relatively brief periods of time between operations. This also means that rapid movement of the solenoid plunger is extremely important. Hence, speed and low power loss become far more significant. The high initial voltage V,, which may be twice the minimum actuation voltage, may be applied to speed the plunger movement and yet this will not damage the solenoid coil because a much lower than minimum voltage is applied during the remainder of the actuation period.

it is to be understood that while the detailed drawings and specific examples given describe preferred embodiments of my invention, they are for the purposes of illustration only, that the apparatus of the invention is not limited to the precise details and conditions disclosed and that various changes may be made therein without departing from the spirit of the invention which is defined by the following claims.

What is claimed is:

1. A solenoid drive circuit for energizing and de-energizing a solenoid in response to suitable signals at a drive circuit input, said circuit comprising:

a. a first electronic switch means in series with said solenoid and a source of driving power, for switching current through the solenoid, the first switch means including and being controlled at said drive circuit input;

b. an electronic valve means connected in series with the solenoid at one solenoid terminal for variably controlling a portion of the current through said solenoid, the valve means having a control input;

c. a timing means for providing a time changing current for controlling said valve means and comprising serially connected timing resistance, capacitance, and a diode connected to permit current flow charging the capacitance, said timing means connected between the control input of said valve means and a source of d.c. electrical power; and

d. capacitance discharge means for at times discharging the capacitance and comprising a second electronic switch means connected across said capacitance, the control input of the second electronic switch means connected to have at least the voltage across said diode applied to the control input of the second electronic switch means.

2. A solenoid drive circuit according to claim 1 wherein said source of d.c. electrical power is at the other solenoid terminal and a dissipating resistance is serially connected to said second electronic switch.

3. A circuit according to claim 2 wherein there is provided a holding current conducting means, connected parallel to said valve means for permitting a minimum holding current to flow through the solenoid, bypassing the valve means.

4. A drive circuit according to claim 2 wherein an input resistance is connected across the input terminals of said valve means.

5. A drive circuit according to claim 4 wherein the valve means is a field effect transistor having its gate connected to said charging circuit and the charging current flows substantially through said input resistance.

6. A drive circuit according to claim 2 wherein the valve means comprises a bipolar transistor connected in a common emitter configuration and wherein the timing circuit charging current is substantially emitter-base current of said bipolar transistor.

7. A drive circuit according to claim 2 wherein a. said series capacitance and diode share a common node;

and

b. said second electronic switch is a transistor having its common terminal connected to said common node and its input terminal connected to the opposite side of the diode.

8. A drive circuit according to claim 7 wherein a. the valve means comprises a bipolar transistor connected in a common emitter configuration and wherein the timing circuit charging current is substantially emitter base current of said bipolar transistor;

b. a leakage resistance is connected between the emitter and base terminals of said bipolar transistor; and

c. a holding current conducting means is connected parallel to said valve means for permitting a minimum holding current to flow through the solenoid by passing the valve means.

9. A circuit according to claim 8 wherein a. said first electronic switch means is a darlington amplifier connected between a common terminal of said power sup ly and said solenoid; and b. sat holding current conducting means 18 a resistor having a resistance approximately equal to E /1 R, where R, is the solenoid d.c. resistance, E is the voltage of said source and I,,,,,, is the minimum solenoid holding current. 10. A circuit according to claim 9 wherein said timing capacitance is approximately in the range of l microfarad to 4 microfarads and said timing resistance is approximately 2200 ohms. 

1. A solenoid drive circuit for energizing and de-energizing a solenoid in response to suitable signals at a drive circuit input, said circuit comprising: a. a first electronic switch means in series with said solenoid and a source of driving power, for switching current through the solenoid, the first switch means including and being controlled at said drive circuit input; b. an electronic valve means connected in series with the solenoid at one solenoid terminal for variably controlling a portion of the current through said solenoid, the valve means having a control input; c. a timing means for providing a time changing current for controlling said valve means and comprising serially connected timing resistance, capacitance, and a diode connected to permit current flow charging the capacitance, said timing means connected between the control input of said valve means and a source of d.c. electrical power; and d. capacitance discharge means for at times discharging the capacitance and comprising a second electronic switch means connected across said capacitance, the control input of the second electronic switch means connected to have at least the voltage across said diode applied to the control input of the second electronic switch means.
 2. A solenoid drive circuit according to claim 1 whErein said source of d.c. electrical power is at the other solenoid terminal and a dissipating resistance is serially connected to said second electronic switch.
 3. A circuit according to claim 2 wherein there is provided a holding current conducting means, connected parallel to said valve means for permitting a minimum holding current to flow through the solenoid, bypassing the valve means.
 4. A drive circuit according to claim 2 wherein an input resistance is connected across the input terminals of said valve means.
 5. A drive circuit according to claim 4 wherein the valve means is a field effect transistor having its gate connected to said charging circuit and the charging current flows substantially through said input resistance.
 6. A drive circuit according to claim 2 wherein the valve means comprises a bipolar transistor connected in a common emitter configuration and wherein the timing circuit charging current is substantially emitter-base current of said bipolar transistor.
 7. A drive circuit according to claim 2 wherein a. said series capacitance and diode share a common node; and b. said second electronic switch is a transistor having its common terminal connected to said common node and its input terminal connected to the opposite side of the diode.
 8. A drive circuit according to claim 7 wherein a. the valve means comprises a bipolar transistor connected in a common emitter configuration and wherein the timing circuit charging current is substantially emitter base current of said bipolar transistor; b. a leakage resistance is connected between the emitter and base terminals of said bipolar transistor; and c. a holding current conducting means is connected parallel to said valve means for permitting a minimum holding current to flow through the solenoid by passing the valve means.
 9. A circuit according to claim 8 wherein a. said first electronic switch means is a darlington amplifier connected between a common terminal of said power supply and said solenoid; and b. said holding current conducting means is a resistor having a resistance approximately equal to Ecc/Imin - Rs where Rs is the solenoid d.c. resistance, Ecc is the voltage of said source and Imin is the minimum solenoid holding current.
 10. A circuit according to claim 9 wherein said timing capacitance is approximately in the range of 1 microfarad to 4 microfarads and said timing resistance is approximately 2200 ohms. 